One unique quality of travel aboard a space vehicle, such as the Space Shuttle, is the feeling that occurs due to weightlessness, which few persons are privileged to experience. Within the artificial atmosphere existent in the confines of the space vehicle, persons, objects and materials can simply float in space. In more scientific terms, the environment in an ideal spacecraft is referred to as a microgravity environment, one in which the acceleration experienced by a mass, such as the human body, tools and like, is reduced to the order of one millionth or less of the level found at sea level at the earth's equator, 9.8.times.10.sup.-6 meters per second per second, relative to the spacecraft.
A microgravity environment has long been recognized as an ideal environment to carry on certain types of experiments and manufacture, which, due to gravity, cannot be carried out on earth. As example, furnaces, crystal growth modules, biological experimental apparatus, combustion and mixing facilities, and materials science investigations, including but not limited to semiconductors, glass amorphous solids, high temperature alloys, ceramics, fluid and combustion physics; biotechnology, including protein crystal growth, separation of biological products, and controlled microgravity experiments, should all benefit in a microgravity environment. To further explore the reasons for the desirability of a gravity-free environment for such experiments and manufactures, the reader may refer to the technical literature for additional details.
Theoretically thus, crystal-growing apparatus and other equipment may be placed in a corner of the space vehicle and allowed to function in a microgravity environment unimpeded. However, transient force effects produced in practice during space vehicle operation interferes with the microgravity; a practical difficulty inherent in space vehicle operation. Astronauts moving about the space vehicle and bumping into or pushing themselves off the space craft's walls create a reaction in the space vehicle; bumping into or shoving off of the equipment itself creates a reaction in the equipment. Performing required exercise on the treadmill, carried in the space vehicle as a physical health measure, produces vibration. Electric motors from time to time are actuated to adjust the position of the spacecraft's solar arrays. That motor operation creates a torque and that torque produces a counter torque on the spacecraft. Each such action produces an equal and opposite reaction, an elementary law of Newtonian physics.
Such shock and vibration are acceleration forces. If the sensitive equipment is thereby momentarily accelerated, that acceleration simulates a gravitational effect, often one that is greater than 9.8.times.10.sup.-6 meters (per second).sup.2. Thus during the period when all the astronauts are at work, the "work day", the environment in practice is one of only milligees of gravitation, one one-thousandths of earth's gravitational acceleration, several orders of magnitude higher than the ideal. Only when the astronauts are all at rest does the environment consistently achieve a lower level of gravitational effect, such as those produced from time to time by the solar array adjustment motors, interrupted by periods of microgravity.
In a practical spacecraft, the term microgravity is given a more expanded definition which allows for accelerations of microgee levels or below at frequencies of 0.1 Hz or less and increases from that level linearly to milligee levels at between 0.1 Hz and 100 Hz, the latter being the levels produced by the solar array motors and the like. In practice thus, in spacecraft useage, and as used herein, the term microgravity is intended to encompass such an expanded meaning.
While all such microgee and milligee forces are minute by standards on earth, they are significant enough as compared to microgravity levels to jeopardize the results obtained from the on-board experiment or manufacture. A need exists to isolate those manufacturing apparatus requiring a microgravity environment on board the spacecraft from such externally created acceleration forces; to stabilize the manufacturing equipment.
Accordingly an object of the present invention is to enable manufacturing processes and experiments to be carried out in a microgravity environment.
Another object is to isolate selected equipment carried within the environment inside an orbiting space vehicle from large accelerating forces, in excess of the levels of microgravity, over extended periods.
An additional object is to provide a non-contacting active vibration isolation system and a stabilized platform for use within an orbiting space vehicle.
The subject of vibration isolation is not new. Others, including the assignee of the present invention, have heretofore provided active isolation devices for stabilizing aiming and tracking devices used in helicopters, recognized as a high angular vibration environment. Active isolation devices for stabilizing aiming and tracking devices has also found use in space vehicles, a low vibration environment, but in which distance to a potential object being monitored are great, in which the effect of such low vibrations on the target is magnified. Such systems must permit the aiming device to be moved and be pointed at a target and then allow that target to be tracked over a limited period of time. With vibrations isolated to a great degree, a steady aim is possible.
As one example, the McDonnell Douglas Company, Huntington Beach, Calif., assignee of the present invention, markets and sells a stabilized aiming device, referred to as the "McDonnell Douglas Mast Mounted Sight" that is mounted atop the rotor mast on helicopters. That aiming device allows the helicopter pilot to obtain a stable television picture of a distant target, despite the inherent vibrations encountered in helicopter operation.
As another example U.S. Pat. No. 4,848,525 granted Jul. 18, 1989 to Jacot et al for a Dual Mode Vibration Isolator ("Jacot") describes a laser targeting and aiming device intended for use in space vehicles, wherein like the helicopters, transmission of vibrations to the aiming device must be minimized while allowing the laser to be moved. For this Jacot employs a combination of narrow gap magnetic actuators and linear actuators. Jacot notes a prior proposal to use magnetic actuators to support one body relative to another by magnetic fields, citing NASA Contractor Report 3343, entitled "Design of the Annular Suspension and Pointing System", October 1980 in which the actuator's stator is suspended between the pole faces of the stator cores by the magnetic field. Further, Jacot notes that the magnetic actuators in such proposal must contain very large gaps between the pole portions and requires large stators as requires large currents and weight. Jacot concludes that employing wide gap magnetic actuators in on-board orbiting systems is a significant disadvantage.
Applicants recognized that design concerns for a target and aiming systems, which accomplish an assigned function in a short period of time, are not identical with those for a microgravity manufacturing environment, wherein manufacturing is carried out over long periods, such as a day, a week or longer. Even so, the present applicants believe some aspects of vibration isolation in the former kind of apparatus, particularly, the wide gap magnetic actuators, can be employed to advantage in the latter microgravity environment, envisioning an advantage in minimizing the time and expense in design and manufacture. Wide gap magnetic actuators and the associated control electronics are of proven capability and are available essentially off-the-shelf.
A still further object of the invention, therefore, is to provide an easily manufactured non-contacting active vibration isolation system and associated stabilized platform for orbiting space vehicle microgravity manufacturing operations that incorporates wide gap magnetic actuators.